This invention relates generally to VAR generators and more specifically to VAR generators employing thyristor switched capacitors.
Early VAR generator designs utilized the concept of a thyristor controlled inductor and a fixed or mechanically switched capacitor network to provide power factor correction for industrial loads such as melting furnaces. Examples of this approach can be found in U.S. Pat. No. 3,936,727, "High Speed Control of Reactive Power for Voltage Stabilization in Electric Power Systems" issued Feb. 3, 1976 to Kelley, Jr. et al.; U.S. Pat. No. 4,047,097, "Apparatus and Method for Transient Free Energization and Deenergization of Static VAR Generators" issued Sept. 6, 1977 to Gyugyi et al.; U.S. Pat. No. 3,999,117, "Method and Control Apparatus for Static VAR Generator and Compensator" issued Dec. 21, 1976 to Gyugyi et al.; and, U.S. Pat. No. 4,274,135, "Gating Circuit for High Voltage Thyristor Strings" issued June 16, 1981 to Rosa et al. With the advent of using VAR generators for compensating transmission lines the use of thyristor controlled inductors with fixed capacitors fell into disfavor because of the high power losses, typically one percent of the system power, associated with these circuits. As a result, more efficient VAR generators using thyristor switched capacitors with thyristor controlled inductors were developed.
Examples of thyristor switched capacitors in static VAR generators may be found in U.S. Pat. No. 4,307,331 "Hybrid Switched-Capacitor Controlled-Inductor Static VAR Generator and Control Apparatus" issued Dec. 22, 1981 to Gyugyi and U.S. Pat. No. 4,234,843 "Static VAR Generator with Discrete Capacitive Current Levels" issued Nov. 18, 1980 to Gyugyi et al. and U.S. Pat. No. 4,104,576, "Equipment With Controlled Reactance" issued Aug. 1, 1978 to Frank. In these types of static VAR generators, a capacitor bank is formed from a number of capacitors, each one typically in series with a bidirectional thyristor switch and a current limiting inductor. In schemes such as may be found in the above-mentioned patents, the thyristor switches are normally fired in response to a VAR demand signal at the times when capacitor voltage and the AC network voltage are equal, that is when the voltage across the thyristor switches is zero. However, the disconnection of the capacitor banks takes place at the instant when their current becomes zero. At these instants of time the voltage across the capacitor bank is equal to the peak of the AC network voltage. After disconnection, the capacitor bank remains charged to that voltage. Because the capacitor bank remains charged to the peak of the AC network voltage applied, the voltage across the thyristor switch will be the difference between the applied AC system voltage and the capacitor charge voltage. This difference reaches a maximum value of twice the peak AC voltage once in each cycle. As a result at a minimum the thyristor switch must be able to withstand or block this voltage.
The necessity to block two times the peak AC voltage will not normally present a problem in maintaining thyristor switch integrity. However, under some conditions of the AC supply network, the AC voltage may transiently increase well above it nominal peak values to excessively high voltage levels. Should the capacitor banks be disconnected when this high level voltage is present, both the capacitor bank and the thyristor switch would be subjected to an excessively high voltage. One solution which is known in the art is to utilize a non-linear clamping device connected across the thyristor switch. For reliability the breakover voltage level of a present day non-linear clamping device is designed to be approximately twice as high as the normal maximum operating voltage they would be expected to encounter. For a thyristor switched capacitor normal maximum voltage would be twice the normal peak voltage, making the breakover voltage about 4 times the normal peak voltage. Thus the maximum residual voltage across a capacitor bank would remain high and the maximum voltage across the thyristor switch can be as great as four times the peak voltage level encountered in normal operation. Therefore, under severe overvoltage conditions utilizing the present art such as may be found in the above-mentioned patents, both the capacitor bank and the thyristor switch typically would be subjected to twice the normal operating voltage stresses. Further, if a thyristor is fired either intentionally or unintentionally, a heavily overcharged capacitor could be reconnected to the AC network which may result in a very large surge current through the thyristor switch and a substantial transient disturbance in an AC network.
As a result of the overvoltage conditions which can occur with the thyristor switched capacitor various protection schemes have been proposed. In U.S. Pat. No. 3,731,183, "Power Control and Phase Angle Correcting Apparatus" issued May 1, 1973 to Johnson et al., a non-linear clamping device to limit voltage transients occurring across the secondary of the coupling transformer and is in effect in parallel with all the circuit branches, each circuit branch composed of a thyristor switch, a capacitor and an inductor in series connection. However, with this arrangement discharge of the capacitor in any manner is not possible. Only the maximum voltage applied to the whole compensating system is limited using this approach. In U.S. Pat. No. 4,075,510, "Thyristor Voltage Switching Circuits" issued Feb. 21, 1978 to Pascente, the use of a non-linear clamping device across a semi-conductor switch is shown as a prior art protection means. This arrangement was believed by Pascente to be unreliable because in the resistive inductive load circuit considered by Pascente the clamping device would be subjected to repeated voltage surges and may burn out. There, in order to avoid the destruction of the clamping device during overvoltage conditions, the semi-conductor switch is turned on to shunt the clamping device instead of allowing the clamping device to limit the overvoltage on the switch. Thus the switch acts as a protective device for the clamping device, not the converse.
In U.S. Pat. No. 4,274,135, "Gating Circuit For High Voltage Thyristor Strings" issued June 16, 1981 to Rosa et al., the protection of the thyristor switches is achieved by being able to keep the thyristors in full conduction continuously during system overvoltages. There the thyristor switch is brought into full conduction and the reactive impedance in series with the thyristor switch is subjected to the system overvoltage. The reactive impedance conducts heavy currents during the overvoltage intervals. However, this protection method is not practical if the reactive impedance is capacitive because the connection of a charged capacitor to a power system would aggravate overvoltage conditions. In fact, it is known that capacitors are disconnected during overvoltages to prevent this problem. Other examples of protection circuits for thyristor switches are shown in U.S. Pat. No. 3,947,726, "Reverse Voltage Surge Protection for High Voltage Thyristors" issued Mar. 30, 1976 to DeCecco et al.; U.S. Pat. No. 3,943,427, "Apparatus for Protecting the Thyristors of a High Voltage Convertor From Overvoltage" issued Mar. 9, 1976 to Tolstov et al.; and, U.S. Pat. No. 4,282,568, "Electric Power Converting Apparatus" issued Aug. 4, 1981 to Kobayashi et al. In general these three patents deal with the protection of unidirectional thyristor switches used in AC/DC power converters not the bidirectional thyristor switches used in VAR generators considered in the present invention. In DeCecco non-linear clamping devices are used to protect individual thyristors in a high voltage thyristor switch against reverse voltage transients generated when the thyristor switches turn off. The non-linear clamping device is connected in series with an auxiliary thyristor switch, the combination in shunt with each thyristor of the whole switch. The auxiliary thyristor is triggered into conduction at some voltage level below the avalanche breakdown voltage of the main thyristor in the thyristor switch thereby connecting the clamping device in parallel with the main thyristor. This method is essentially an elaborate snubber circuit arrangement to handle the dynamic voltage transients and voltage sharing for each element of the unidirectional thyristor switch. However, this approach is inapplicable when used with thyristor switched capacitors because these thyristors are not subjected to high voltage transients at the instant of turn off. In Tolstov et al. the protection circuit utilizes an auxiliary clamping circuit to protect non-linear clamping devices that are connected in parallel with each thyristor or thyristor group. A sensing circuit detects high current in a chain of individual clamping devices and activates an auxiliary clamping circuit to limit the voltage below the total breakover voltage represented by the sums of the individual clamping devices. However, this protection scheme is utilized in AC to DC power converters and is not usable for the controlled discharge of thyristor switched capacitors. In Kobayashi et al. non-linear resistors are connected in parallel with individual thyristors or thyristor groups of a thyristor switch composed of series connected devices and are used in an overall voltage sharing network. With Kobayashi et al. DeCecco et al. and Tolstov et al. the thyristor protection schemes are utilized in AC to DC converters. With these devices the thyristors are used as unidirectional devices and are all turned off simultaneously and therefore unavailable to accomplish controlled discharge of a capacitor bank.
All of these protection schemes fall into one of two categories. The first inhibits the disconnection of the capacitor bank under high AC network voltage conditions (by keeping the thyristor switch conductive). The second uses a metal oxide varistor or surge arrestor across the thyristor switch to limit the overvoltage to an acceptable value. The drawback with the first approach is that the connected capacitor C3 will increase the already high network voltage (due to the leading current it draws through the basically inductive AC network) and can also create dangerous oscillatory conditions in the network which may further aggravate overvoltage problems. With the second approach, the breakover voltage required for reliable operation of the clamping device can result in the capacitor being charged to a voltage having a value up to two times normal peak voltage of the system. This requires that the voltage rating of the thyristor switch be increased suitably to withstand these increased voltages.
One object of the present invention is to provide a reliable method of overvoltage protection for thyristor switched capacitors in a VAR generator. A further objective is to provide controlled discharge of the thyristor switched capacitor banks without requiring an impractically low clamping voltage level for the protected devices. Another objective is to reduce the required voltage surge rating of the thyristor switch assembly thereby providing cost as well as size benefits.